The Secret Banner can be found to the South of Frosty Flights.
Bird's Eye View
The Secret Banner is just directly south of Frosty Flights on a road.
The Secret Banner is just on the road south of Frosty Flights so land there & it should show up in front of you.
Season 7 Secret Banner / Battle Star Locations
Fortnite Battle Royale's 7th Season's Secret Challenges reward you with Secret Battle Stars and Secret Banners! Hurry though as Season 7 ends soon! Don't miss this chance to complete the challenges before Season 8 launches!